144 MARINE INVERTEBRATES 
erease also by self-division. In this case a constriction is formed, 
which gradually deepens and forms a complete partition of the 
body, and two individuals exist where originally there was but one. 
The Actiniaria are carnivorous and very voracious. They feed 
on small organisms and on shell-fish and crustaceans, which they 
suck out of their shells. To secure their prey they are armed 
with an abundant supply of stinging-cells on the tentacles, and 
also with fine stinging-threads which are ejected from pores 
which are distributed over the whole body. 
Sea-anemones vary greatly in color and form, and when ex- 
panded suggest flowers, but do not resemble the one for which 
they are named. They abound on every shore, the same genus 
often being found in widely separated regions. They are larger 
and more highly colored in tropical waters. Many of the species 
are littoral, and are found in the tide-pools of rocky caverns, 
on the under side of rocks, and on the piles of wharves and 
bridges at low-water mark. The majority of them are attached, 
but are able to change their location; others, Hdwardsia and 
Cerianthus, swim about when young, and in the adult state burrow 
in the sand or mud, leaving only their tentacles exposed. Bucid- 
ium parasiticum is parasitic on the folds of the membrane which 
hangs from the mouth of the large jellyfish Cyanea arctica. Fenja 
and Peachia lie on the sea-bottom, with their bodies horizontal 
like a worm, the mouth-end and tentacles erect. 
Adamsia palliata furnishes another example of commensalism ; 
it lives on the back of shells, commonly the whelk, inhabited by 
hermit-crabs. Minyasis pelagic; it has a float at one end, and by 
means of its tentacles swims about freely. 
A very few species only are described below, since these polyps 
are unmistakable, being always columnar bodies, with the upper 
disk more or less crowded with tentacles, and so brilliant in col- 
oring and beautiful in form as to attract attention if found in 
the expanded state; otherwise they are easily overlooked. 
Genus Metridiwm 
M. marginatum. This is the most conspicuous and abundant sea- 
anemone of the northeastern coast. It is common from New York 
